arXiv subjects

Yuichiro Ando

Publications and source records attributed to Yuichiro Ando.

At least 37 records · Page 2Linked to original sources

arXiv2017

Spincaloritronic signal generation in non-degenerate Si

Spincaloritronic signal generation due to thermal spin injection and spin transport is demonstrated in a non-degenerate Si spin valve. The spin-dependent Seebeck effect is used for the spincaloritronic signal generation, and the thermal gradient of about 200 mK at an interface of Fe and Si enables generating a spin voltage of 8 μV at room temperature. A simple expansion of a conventional spin drift-diffusion model with taking into account the spin-dependent Seebeck effect shows semiconductor materials are quite potential for the spincaloritronic signal generation comparing with metallic materials, which can allow efficient heat recycling in semiconductor spin devices.

physics.app-ph↗

arXiv2017

Spin injection into silicon detected by broadband ferromagnetic resonance spectroscopy

We studied the spin injection in a NiFe(Py)/Si system using broadband ferromagnetic resonance spectroscopy. The Gilbert damping parameter of the Py layer on top of the Si channel was determined as a function of the Si doping concentration and Py layer thickness. For fixed Py thickness we observed an increase of the Gilbert damping parameter with decreasing resistivity of the Si channel. For a fixed Si doping concentration we measured an increasing Gilbert damping parameter for decreasing Py layer thickness. No increase of the Gilbert damping parameter was found Py/Si samples with an insulating interlayer. We attribute our observations to an enhanced spin injection into the low-resistivity Si by spin pumping.

physics.acc-ph↗

arXiv2017

Investigation of spin scattering mechanism in silicon channels of Fe/MgO/Si lateral spin valves

The temperature evolution of spin relaxation time, τsf, in degenerate silicon (Si)-based lateral spin valves is investigated by means of the Hanle effect measurements. τsf at 300 K is estimated to be 1.68+-0.03 ns and monotonically increased with decreasing temperature down to 100 K. Below 100 K, in contrast, it shows almost a constant value of ca. 5 ns. The temperature dependence of the conductivity of the Si channel shows a similar behavior to that of the τsf, i.e., monotonically increasing with decreasing temperature down to 100 K and a weak temperature dependence below 100 K. The temperature evolution of conductivity reveals that electron scattering due to magnetic impurities is negligible. A comparison between τsf and momentum scattering time reveals that the dominant spin scattering mechanism in the Si is the Elliott-Yafet mechanism, and the ratio of the momentum scattering time to the τsf attributed to nonmagnetic impurities is approximately 3.77*10^-6, which is more than two orders of magnitude smaller than that of copper.

physics.app-ph↗

arXiv2017

Quantitative investigation of the inverse Rashba-Edelstein effect in Bi/Ag and Ag/Bi on YIG

The inverse Rashba-Edelstein effect (IREE) is a spin conversion mechanism that recently attracts attention in spintronics and condensed matter physics. In this letter, we report an investigation of the IREE in Bi/Ag by using ferrimagnetic insulator yttrium iron garnet (YIG). We prepared two types of samples with opposite directions of the Rashba field by changing a stacking order of Bi and Ag. An electric current generated by the IREE was observed from both stacks, and an efficiency of spin conversion -characterized by the IREE length- was estimated by taking into account a number of contributions left out in previous studies. This study provides a further insight into the IREE spin conversion mechanism: important step towards achieving efficient spin-charge conversion devices.

cond-mat.mes-hall↗

arXiv2017

Strong evidence for d-electron spin transport at room temperature at a LaAlO3/SrTiO3 interface

A d-orbital electron has an anisotropic electron orbital and is a source of magnetism. The realization of a 2-dimensional electron gas (2DEG) embedded at a LaAlO3/SrTiO3 interface surprised researchers in materials and physical sciences because the 2DEG consists of 3d-electrons of Ti with extraordinarily large carrier mobility, even in the insulating oxide heterostructure. To date, a wide variety of physical phenomena, such as ferromagnetism and the quantum Hall effect, have been discovered in this 2DEG systems, demonstrating the ability of the d-electron 2DEG systems to provide a material platform for the study of interesting physics. However, because of both ferromagnetism and the Rashba field, long-range spin transport and the exploitation of spintronics functions have been believed difficult to implement in the d-electron 2DEG systems. Here, we report the experimental demonstration of room-temperature spin transport in the d-electron-based 2DEG at a LaAlO3/SrTiO3 interface, where the spin relaxation length is ca. exceeding 200 nm. Our finding, which counters the conventional understandings to d-electron 2DEGs, opens a new field of d-electron spintronics. Furthermore, this work highlights a novel spin function in the conductive oxide system.

cond-mat.mtrl-sci↗

arXiv2016

Switching of Charge-Current-Induced Spin Polarization in the Topological Insulator BiSbTeSe2

The charge-current-induced spin polarization is a key property of topological insulators for their applications in spintronics. However, topological surface states are expected to give rise to only one type of spin polarization for a given current direction, which has been a limiting factor for spin manipulations. Here we report that in devices based on the bulk-insulating topological insulator BiSbTeSe2, an unexpected switching of spin polarization was observed upon changing the chemical potential. The spin polarization expected from the topological surface states was detected in a heavily electron-doped device, whereas the opposite polarization was reproducibly observed in devices with low carrier densities. We propose that the latter type of spin polarization stems from topologically-trivial two-dimensional states with a large Rashba spin splitting, which are caused by a strong band bending at the surface of BiSbTeSe2 beneath the ferromagnetic electrode used as a spin detector. This finding paves the way for realizing the "spin transistor" operation in future topological spintronic devices.

cond-mat.mes-hall↗

arXiv2016

Transport and spin conversion of multi-carriers in semimetal bismuth

In this paper, we report on the investigation of (1) the transport properties of multi-carriers in semi-metal Bi and (2) the spin conversion physics in this semimetal system in a ferrimagnetic insulator, yttrium-iron-garnet. Hall measurements reveal that electrons and holes co-exist in the Bi, with electrons being the dominant carrier. The results of a spin conversion experiment corroborate the results of the Hall measurement; in addition, the inverse spin Hall effect governs the spin conversion in the semimetal/insulator system. This study provides further insights into spin conversion physics in semimetal systems.

cond-mat.mtrl-sci↗

arXiv2016

Observation of large spin accumulation voltages in non-degenerate Si spin devices due to spin drift effect: Experiments and theory

A large spin-accumulation voltage of more than 1.5 mV at 1 mA, i.e., a magnetoresistance of 1.5 Ω, was measured by means of the local three-terminal magnetoresistance in nondegenerate Si-based lateral spin valves (LSVs) at room temperature. This is the largest spin-accumulation voltage measured in semiconductor-based LSVs. The modified spin drift-diffusion model, which successfully accounts for the spin drift effect, explains the large spin-accumulation voltage and significant bias-current-polarity dependence. The model also shows that the spin drift effect enhances the spin-dependent magnetoresistance in the electric two terminal scheme. This finding provides a useful guiding principle for spin metal-oxide semiconductor field-effect transistor (MOSFET) operations.

cond-mat.mtrl-sci↗

arXiv2015

Room-temperature operation of Si spin MOSFET with high on/off spin signal ratio

We experimentally demonstrate a Si spin metal-oxide-semiconductor field-effect transistor (MOSFET) that exhibits a high on/off ratio of source-drain current and spin signals at room temperature. The spin channel is non-degenerate n-type Si, and an effective application of gate voltage in the back-gated structure allows the spin MOSFET operation. This achievement can pave the way to practical use of the Si spin MOSFET.

cond-mat.mes-hall↗

arXiv2015

Spin transport and spin conversion in compound semiconductor with non-negligible spin-orbit interaction

A quantitative investigation of spin-pumping-induced spin-transport in n-GaAs was conducted at room temperature (RT). GaAs has a non-negligible spin orbit interaction, so that electromotive force due to the inverse spin Hall effect (ISHE) of GaAs contributed to the electromotive force detected with a platinum (Pt) spin detector. The electromotive force detected by the Pt spin detector had opposite polarity to that measured with a Ni80Fe20/GaAs bilayer due to the opposite direction of spin current flow, which demonstrates successful spin transport in the n-GaAs channel. A two-dimensional spin-diffusion model that considers the ISHE in the n-GaAs channel reveals an accurate spin diffusion length of t_s = 1.09 um in n-GaAs (NSi = 4x10^16 cm-3) at RT, which is approximately half that estimated by the conventional model.

cond-mat.mtrl-sci↗

arXiv2015

An experimental demonstration of room-temperature spin transport in n-type Germanium epilayers

We report the first experimental demonstration of room-temperature spin transport in n-type Ge epilayers grown on a Si(001) substrate. By utilizing spin pumping under ferromagnetic resonance, which inherently endows a spin battery function for semiconductors connected with the ferromagnet, a pure spin current is generated in the n-Ge at room temperature. The pure spin current is detected by using the inverse spin Hall effect of either Pt or Pd electrode on the n-Ge. A theoretical model including a geometrical contribution allows to estimate a spin diffusion length in n-Ge at room temperature to be 660 nm. The temperature dependence of the spin relaxation time provides evidence for Elliott-Yafet spin relaxation mechanism.

cond-mat.mtrl-sci↗

arXiv2014

Electrical detection of the spin polarization due to charge flow in the surface state of the topological insulator Bi_1.5 Sb_0.5 Te_1.7 Se_1.3

We detected the spin polarization due to charge flow in the spin non-degenerate surface state of a three dimensional topological insulator by means of an all-electrical method. The charge current in the bulk-insulating topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) was injected/extracted through a ferromagnetic electrode made of Ni80Fe20, and an unusual current-direction-dependent magnetoresistance gives evidence for the appearance of spin polarization which leads to a spin-dependent resistance at the BSTS/Ni80Fe20 interface. In contrast, our control experiment on Bi2Se3 gave null result. These observations demonstrate the importance of the Fermi-level control for the electrical detection of the spin polarization in topological insulators.

cond-mat.mtrl-sci↗

arXiv2014

Observation of spin-charge conversion in CVD-grown single-layer graphene

Conversion of pure spin current to charge current in single-layer graphene (SLG) is investigated by using spin pumping. Large-area SLG grown by chemical vapor deposition is used for the conversion. Efficient spin accumulation in SLG by spin pumping enables observing an electromotive force produced by the inverse spin Hall effect (ISHE) of SLG. The spin Hall angle of SLG is estimated to be 6.1*10-7. The observed ISHE in SLG is ascribed to its non-negligible spin-orbit interaction in SLG.

cond-mat.mtrl-sci↗

arXiv2014

Spin transport in non-degenerate Si with a spin MOSFET structure at room temperature

Spin transport in non-degenerate semiconductors is expected to pave a way to the creation of spin transistors, spin logic devices and reconfigurable logic circuits, because room temperature (RT) spin transport in Si has already been achieved. However, RT spin transport has been limited to degenerate Si, which makes it difficult to produce spin-based signals because a gate electric field cannot be used to manipulate such signals. Here, we report the experimental demonstration of spin transport in non-degenerate Si with a spin metal-oxide-semiconductor field-effect transistor (MOSFET) structure. We successfully observed the modulation of the Hanle-type spin precession signals, which is a characteristic spin dynamics in non-degenerate semiconductor. We obtained long spin transport of more than 20 μm and spin rotation, greater than 4π at RT. We also observed gate-induced modulation of spin transport signals at RT. The modulation of spin diffusion length as a function of a gate voltage was successfully observed, which we attributed to the Elliott-Yafet spin relaxation mechanism. These achievements are expected to make avenues to create of practical Si-based spin MOSFETs.

cond-mat.mtrl-sci↗

arXiv2014

Self-induced inverse spin Hall effect in permalloy at room temperature

Inverse spin Hall effect (ISHE) allows the conversion of pure spin current into charge current in nonmagnetic materials (NM) due to spin-orbit interaction (SOI). In ferromagnetic materials (FM), SOI is known to contribute to anomalous Hall effect (AHE), anisotropic magnetoresistance (AMR), and other spin-dependent transport phenomena. However, SOI in FM has been ignored in ISHE studies in spintronic devices, and the possibility of "self-induced ISHE" in FM has never been explored until now. In this paper, we demonstrate the experimental verification of ISHE in FM. We found that the spin-pumping-induced spin current in permalloy (Py) film generates a transverse electromotive force (EMF) in the film itself, which results from the coupling of spin current and SOI in Py. The control experiments ruled out spin rectification effect and anomalous Nernst effect as the origin of the EMF.

cond-mat.mes-hall↗

arXiv2014

Exchange-driven magnetoresistance in silicon facilitated by electrical spin injection

We use electrical spin injection to probe exchange interactions in phosphorus doped silicon (Si:P). The detection is enabled by a magnetoresistance effect that demonstrates the efficiency of exchange in imprinting spin information from the magnetic lead onto the localized moments in the Si:P region. A unique Lorentzian-shaped signal existing only at low temperatures ($\lesssim 25 K$) is observed experimentally and analyzed theoretically in electrical Hanle effect measurement. It stems from spin-dependent scattering of electrons by neutral impurities in Si:P. The shape of this signal is not directly related to spin relaxation but to exchange interaction between spin-polarized electrons that are localized on adjacent impurities.

cond-mat.mtrl-sci↗

arXiv2014

Spin Drift in Highly Doped n-type Si

A quantitative estimation of spin drift velocity in highly doped n-type silicon (Si) at 8 K is presented in this letter. A local two-terminal Hanle measurement enables the detection of a modulation of spin signals from the Si as a function of an external electric field, and this modulation is analyzed by using a spin drift-diffusion equation and an analytical solution of the Hanle-type spin precession. The analyses reveal that the spin drift velocity is linearly proportional to the electric field. The contribution of the spin drift effect to the spin signals is crosschecked by introducing a modified nonlocal four-terminal method.

cond-mat.mtrl-sci↗

arXiv2014

Local magnetoresistance in Fe/MgO/Si lateral spin valve at room temperature

Room temperature local magnetoresistance in two-terminal scheme is reported. By employing 1.6 nm-thick MgO tunnel barrier, spin injection efficiency is increased, resulting in large non-local magnetoresistance. The magnitude of the non-local magnetoresistance is estimated to be 0.0057 ohm at room temperature. As a result, a clear rectangle signal is observed in local magnetoresistance measurement even at room temperature. We also investigate the origin of local magnetoresistance by measuring the spin accumulation voltage of each contact separately.

cond-mat.mtrl-sci↗